added more file uploads test for better coverage

Meta: [s3 links default to https](http://meta.discourse.org/t/s3-links-default-to-https/5758/9)

- [x] Added tests for image uploads to provide better coverage of the image upload feature.
Those 3 tests are pretty basic but should cover the happy-path.

- [x] Removed the `config/fog_credentials.yml.sample` file and extracted its settings into the following site settings: `s3_access_key_id`, `s3_secret_access_key` and `s3_region`.
I find it better to have them in the site settings as they are now more *local* to the `enable_s3_uploads` setting.

- [x] Refactored `create_on_s3` a bit and changed the format of the url used to store images on s3 (previous format felt a bit noisy and provided no useful information).